Obsdian had a special problem with PoE and Deadfire when it comes to RTwP: as soon as "mundane" classes like fighters, rangers etc. received more active abilities the RTwP combat became more and more convoluted since you had to micromanage every character. In BG I/II there are often several party members who just ran around and hit with auto-attacks. Only casters needed special micromanagment. That led to the bahavior in PoE that players - after reaching certain levels of XP and game mechanics expertise - tended to just select all characters, attacked one enemy until dead, rinse and repeat - because the alternative (micromanage every char) became too tedious and complicated. Also more active abilities meant more VFX on screen which didn't go too well with the RTwP approach: multiple players complaied that they couldn't really keep track (visually) what was happening. THis was one of the reasons why they reduced party size from 6 to 5 in Deadfire and put mor work into a sophisticated player-AI scripting tool. This is pulled from conversations and interviews or talks with/of Obsidian devs.
Turn Based has other disadvantages (like taking potentially longer for example or the tendency of feeling too much like a puzzle/being too abstract), but those problems I mentioned above it has not.
And for the record: I am a die-hard PoE/Deadfire player and an old BG fan and only played them with RTwP mode, not Turn Based. But I also played a lot of TB games and find them great, too. Depends on the game and how well everything fits together or how the encounters and combat mechanics are designed. So pleaso no comments like "no real BG fan/hates RTwP".
Josh Sawyer based a lots of POE class design on 5e/4e, so that series is a good example of what 5e in RTwP would look like.
As for TB being longer, I played Deadfire in both RTwP and TB and with all the pausing I had to do in RTwP even with the AI (I admit I didn't make anything complex), TB wasn't any longer on the same difficulty outside a few of the larger boat fights (too many bodies). Some Boss fights were even shorter than in RTwP. I also found using spellcaster much more entertaining in TB and used consumables more often as speed wasn't as much of factor anymore.